A continuing concern of the manufacture of galvanic cells is that electrolyte may creep through a sealed interface of the cell and leak out of the cell. Electrolyte leakage can shorten cell life and can also cause a corrosive deposit to form on the exterior surface of the cell which detracts from the cell's appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Electrolyte leakage occurs in cell systems having aqueous or nonaqueous electrolytes, such as organic solvent-based electrolytes and liquid inorganic cathode-electrolytes, for example those based on thionyl chloride and sulfuryl chloride. Electrolytes such as alkaline electrolytes have an affinity for wetting metal surfaces and are known to creep through a sealed interface of a galvanic cell.
In the prior art it has been a conventional practice to incorporate an insulating gasket between the cell container and cover so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and the cell environment. In addition, it has to be sufficiently flexible and resistant to cold flow under pressure of the seal and to be able to maintain these characteristics so as to insure a proper seal during long periods of storage.
It has been recognized in the prior art that gaskets having fluorine atoms on their surface are more effective in preventing electrolyte creepage. However, while materials such as polytetrafluoroethylene and ethylene-tetrafluoroethylene copolymer have been employed in cell gaskets, these compounds do not possess as desirable a resistance to cold flow under pressure as do other gasket materials such as nylon, polypropylene and the like. Cold flow is defined as the distortion of a solid under sustained pressure with an accompanying inability to return to its original dimensions when the pressure is removed. The cold flow for a given thermoplastic material is related to such thermoplastic material's apparent flexural modulus. The greater the modulus at a constant stress, the lesser the elongation and, in reference to compressive loads, the lesser the tendency to relax while under load at a specific temperature. The apparent flexural modulus for materials such as nylon, polypropylene and high density polyethylene is substantially greater than for materials such as ethylene-tetrafluoroethylene copolymer and polytetrafluoroethylene.
Based on such apparent flexural moduli, it is apparent that nylon will have less of a tendency to relax in compression than will polytetrafluoroethylene and ethylene-polytetrafluoroethylene copolymer. Consequently, nylon and similiar materials such as polypropylene will provide more effective seals than will fluoropolymer gaskets. Moreover, such fluoropolymers are relatively expensive and thus are undesirable from a commercial standpoint.
Thus, it would be desirable to provide a cell having a seal which possesses the electrolyte-creepage resistance of fluoropolymers coupled with the superior effective sealing ability of nylon and polypropylene gaskets.
Japanese Patent Application 90146/1978 discloses the formation of a fluoropolymer film by sputtering or plasma deposition onto the surface of electrochemical cells in order to reduce electrolyte leakage. However, this application is directed to the deposition of a polymeric layer of fluoropolymer material on such surfaces as it indicates that the thickness of the film deposited should be at least 3000 Angstroms in order to avoid pinholes.
It is known in the art of plasma deposition of saturated fluorocarbon materials that hydrogen gas must be incorporated into the plasma reaction mixture along with the saturated fluorocarbon compound in order for a polymeric layer of the thickness of more than 3000 Angstroms to be deposited in a reasonable time frame. As is noted by E. Kay and A. Dilks, Plasma polymerization of fluorocarbons in rf capacitively coupled diode system, J. Vac. Sci. Technol., 18(1), Jan./Feb. 1981 at page 9, "Hydrogen will scavenge F atoms and tie them up as relatively stable HF thereby reducing the F/(CF.sub.2).sub.n ratio and pushing the plasma in the direction of polymerization . . . " Thus, it is evident to one skilled in the art that Japanese Patent Application 90146/1978 would require plasma treatment with a fluorocarbon/hydrogen mixture in order to deposit such a polymeric layer.
It is noteworthy that as polymeric films, such as those deposited in Japanese Application 90146/1978, become thicker they tend to become more brittle so as to cause cohesive failure when subjected to cell sealing forces. Consequently, it would be desirable if the sealing surfaces of a cell could be treated so as to provide the electrolyte repellence of such fluoropolymeric layers accompanied by enhanced adhesion to the sealing surface.
It is therefore an object of this invention to provide a galvanic cell wherein at least a portion of the sealing surfaces of the gasket exhibit the increased electrolyte repellence associated with surfacial fluorine atoms.
It is a further object of this invention to provide a galvanic cell employing a sealing gasket having desirable resistance to cold flow which cell exhibits increased resistance electrolyte leakage.
It is further object of this invention to provide a process for treating at least a portion of at least one sealing surface of a galvanic cell's compressible gasket such that the assembled cell will demonstrate increased resistance to electrolyte leakage.
The foregoing and additional objects of this invention will become apparent from the following description and accompanying drawings and examples.